Foreword
Over the past two decades, the realm of ultrafast science has become vast and exciting and has impacted many areas of chemistry, biology and physics, and other
fields such as materials science, electrical engineering, and optical communication.
The explosive growth in molecular science is principally for fundamental reasons.
In femtochemistry and femtobiology, chemical bonds form and break on the femtosecond time scale, and on this scale of time we can freeze the transition states
at configurations never before seen. Even for nonreactive physical changes, one is
observing the most elementary of molecular processes. On a time scale shorter than
the vibrational and rotational periods, the ensemble behaves coherently as a singlemolecule trajectory.
But these developments would not have been possible without the advent of new
light sources and equally important the crystallization of some key underlying concepts that were in the beginning shrouded in fog. First was the issue of the “uncertainty principle”, which had to be decisively clarified. Second was the question
of whether one could sustain wave packet motion at the atomic scale of distance.
In other words, would the de Broglie wavelength of the atom become sufficiently
short to define classical motion—“classical atoms”—and without significant quantum spreading? This too had to be clearly demonstrated and monitored in the course
of change, not only for elementary processes in molecular systems, but also during
complex biological transformations. And, finally, some questions about the uniqueness and generality of the approach had to be addressed. For example, why not
deduce the information from high-resolution frequency-domain methods and then
Fourier transform to obtain the dynamics? It is surely now clear that transient species
cannot be isolated this way, and that there is no substitute for direct real-time observations that fully exploit the intrinsic coherence of atomic and molecular motions.
Theory has enjoyed a similar explosion in areas dealing with ab initio electronic structures, molecular dynamics, and nonlinear spectroscopies. There has been
progress in calculating potential energy surfaces of reactive systems, especially in
their ground state. On excited-state surfaces, it is now feasible to map out regions
of the surface where transition states and conical intersections are important for the
outcome of change. For dynamics, new methods have been devised for direct viewv
Over the past two decades, the realm of ultrafast science has become vast and exciting and has impacted many areas of chemistry, biology and physics, and other
fields such as materials science, electrical engineering, and optical communication.
The explosive growth in molecular science is principally for fundamental reasons.
In femtochemistry and femtobiology, chemical bonds form and break on the femtosecond time scale, and on this scale of time we can freeze the transition states
at configurations never before seen. Even for nonreactive physical changes, one is
observing the most elementary of molecular processes. On a time scale shorter than
the vibrational and rotational periods, the ensemble behaves coherently as a singlemolecule trajectory.
But these developments would not have been possible without the advent of new
light sources and equally important the crystallization of some key underlying concepts that were in the beginning shrouded in fog. First was the issue of the “uncertainty principle”, which had to be decisively clarified. Second was the question
of whether one could sustain wave packet motion at the atomic scale of distance.
In other words, would the de Broglie wavelength of the atom become sufficiently
short to define classical motion—“classical atoms”—and without significant quantum spreading? This too had to be clearly demonstrated and monitored in the course
of change, not only for elementary processes in molecular systems, but also during
complex biological transformations. And, finally, some questions about the uniqueness and generality of the approach had to be addressed. For example, why not
deduce the information from high-resolution frequency-domain methods and then
Fourier transform to obtain the dynamics? It is surely now clear that transient species
cannot be isolated this way, and that there is no substitute for direct real-time observations that fully exploit the intrinsic coherence of atomic and molecular motions.
Theory has enjoyed a similar explosion in areas dealing with ab initio electronic structures, molecular dynamics, and nonlinear spectroscopies. There has been
progress in calculating potential energy surfaces of reactive systems, especially in
their ground state. On excited-state surfaces, it is now feasible to map out regions
of the surface where transition states and conical intersections are important for the
outcome of change. For dynamics, new methods have been devised for direct viewv